Multi section laser diodes are well known in the art and can be switched between different wavelengths. Typically the diode is calibrated at manufacture to determine the correct control currents that should be applied to the laser so as to effect the desired output frequencies from the laser.
One of the first known multi-section laser diodes is a three-section tuneable distributed Bragg reflector (DBR) laser. Other types of multi-section diode lasers are the sampled grating DBR (SG-DBR), the superstructure sampled DBR (SSG-DBR) and the grating assisted coupler with rear sampled or superstructure grating reflector (GCSR). A review of such lasers is given in Jens Buus, Markus Christian Amann, “Tuneable Laser Diodes” Artect House, 1998 and “Widely Tuneable Semiconductor Lasers” ECOC'00. Beck Mason.
FIG. 1 is a schematic drawing of a SG-DBR 10. The laser comprises of back and front reflector sections 2 and 8 with an intervening gain or active section 6 and phase section 4. An anti-reflection coating 9 is usually provided on the front and rear facets of the chip to avoid facet modes. The back and the front reflectors take the form of sampled Bragg gratings 3 and 5. The pitch of the gratings of the back and front reflectors vary slightly to provide a Vernier tuning effect through varying the current supplied to these sections. The optical path length of the cavity can also be tuned with the phase section, for example by refractive index changes induced by varying the carrier density in this section. A more detailed description of the SG-DBR and other tuneable multi-section diode lasers can be found elsewhere Jens Buus, Markus Christian Amann, “Tuneable Laser Diodes” Artect House, 1998.
Multi-section diode lasers are useful in wavelength division multiplexed (WDM) systems. Example applications are as transmitter sources, as wavelength converters in optical cross connects (OXCs) and for reference sources in heterodyne receivers. Typically, WDM systems have channel spacing conforming to the International Telecommunications Union (ITU) standard G692, which has a fixed point at 193.1 THz and inter-channel spacing at an integer multiple of 50 GHz or 100 GHz. An example dense WDM (DWDM) system could have a 50 GHz channel spacing and range from 191 THz to 196 THz (1525-1560 nm).
As these are multi-section lasers they require some calibration before use to determine the correct values of current to achieve each of the desired output wavelengths of the tuneable laser. For example an SG-DBR laser has 4 sections. If each of these sections has a 300 possible values of current (0-90 mA in steps of 0.3 mA) and as each of the sections of the laser are interdependent on the output of the laser there are 300×300×300×300 possible combinations of current that can be applied to the laser.
Added to this the laser must also meet the requirements for line width, SMSR etc. This means that the laser must be calibrated and a lookup table of currents obtained where each entry in the table consists of the currents required to achieve each wavelength in the frequency plan. Each of these entries are called operating points.
The manufacturing process of tuneable lasers is not fully mature and each device will have its own unique wavelength signature which means that each device is sufficiently different to require a full calibration and data from another laser will not work. This means that each device must be fully calibrated to obtain the lookup table and this table must be used with the device when in operation.
Several techniques to obtain this lookup table of information have been published including “Fast Generation of Optimum Operating Points for Tuneable SG-DBR Laser over 1535-1565 nm Range” John Dunne et al. Conference on Lasers and Electroptics (CLEO) Baltimore, May, 1999 p 147-148 proceedings, “Fast Accurate Characterisation of a GCSR laser over the complete EDFA Band” Tom Farrell et al. LEOS'99 November, San Francisco, “Control of widely tuneable SSG-DBR lasers for dense wavelength division multiplexing” Gert Sarlet, G. Morthier, R. Baets J. Lightwave Technol. vol. 18, no. 8, pp 1128-1138, August 2000, and also in patent WO 00/54380. The first publication mentioned above also utilises a measurement set-up such as that shown in FIG. 2. The apparatus comprises a laser 600 which is controlled by current sources and a temperature control element 610. The output of the laser is passed through a first coupler 620, so as to provide a portion of the output to a wavelength meter 630 and a second portion to a second coupler where it is split further. A first portion of the split light output is passed directly to a first photodiode (photodiode A), whereas the second portion passes through a linear filter and the filtered signal is then detected using photodiode B. By dividing the detected voltage level at photodiode B (which is proportional to the amount of light arriving at photodiode A) by the detected voltage at photodiode A, a value is obtained that is proportional to the wavelength of the light emitted by the laser. Either the value measured by the wavelength meter or the value of photodiode B divided by photodiode B can be used as the wavelength of the light emitted by the laser.
While these methods offer solutions to the general concept of calibration, they are over complicated as they involve many operations and parameters (typical numbers for conventional systems is anything between 10 and 20) to guide the calibration process. Inevitably this leads to several parameters that control the calibration and these are sensitive to particular device structures and cannot cope with device variation of a production line. Also these parameters will often be interdependent leading to a multidimensional space to set up the calibration where only a small subset of the possible parameters will provide good calibration results on the tuneable laser. Ideally the calibration should have a small set of parameters that greatly simplifies the calibration and its dependency on particular device characteristics.
There is therefore a need to provide a method that enables constant and accurate results to be obtained so as to provide for a process control of the calibration process.